A known temperature switch comprises an electrically conductive sheath and an electrically conductive wire positioned within the sheath so as to be spaced from (that is to say not in contact with) the sheath. A solid eutectic salt mixture lies between the sheath and the wire and is in contact with both the sheath and the wire. The eutectic salt mixture is selected so as to melt at a desired threshold temperature. At temperatures below the threshold temperature, the solid eutectic salt mixture is poorly electrically conductive (it has a high electrical impedance). However, on melting, the eutectic salt mixture undergoes a large increase in electrical conductivity (that is to say its electrical impedance decreases). In use, the conductive sheath and the conductive wire act as electrical contacts for detecting the change in conductivity/impedance of the eutectic salt mixture which lies between the sheath and the wire. The change in conductivity/impedance is detected by suitable electronics connected to the sheath and the wire.